1. Field of the Invention
This invention relates to new hydroxy functional copolymers containing imide structural units, to a process for the preparation of such copolymers and to their use as binders or binder components for the production of lacquers, coatings, sealants, adhesives or printing inks.
2. Description of the Prior Art
Copolymers of acrylic and vinyl monomers were distinguished by numerous outstanding properties such as resistance to hydrolysis and weathering, resistance to yellowing and gloss retention. Reactive substances which were valuable for the coatings industry were obtained by the incorporation of functional groups such as hydroxyl groups. The products obtained were used in large quantities for lacquers, coatings, adhesives, printing inks and sealing compounds. The introduction of hydroxyl groups is normally achieved by the copolymerization of hydroxy functional monomers such as hydroxyethyl methacrylate. To obtain high crosslinking densities, it is necessary to incorporate relatively large quantities of hydroxy functional monomers, with the result that the amount of variation of the other monomers is restricted and the products obtained were relatively expensive.
Polyimides may be prepared, for example, from diamines and bisanhydrides such as pyromellitic acid bisanhydride. They were distinguished inter alia by their excellent electric insulating properties and high thermostability during continuous use. One serious disadvantage of these substances is their poor solubility which necessitates the use of large quantities of solvents which were not physiologically harmless such as dimethyl acetamide.
A process for the preparation of ethanol imide derivatives from styrene/maleic acid anhydride cotelomers having average molecular weights M.sub.n of from 650 to 1600 is described in DE-OS 1 745 954. This process initially results in cross-linked, gel type products which were converted into low molecular weight ethanol imide products only when heated to 150.degree.-250.degree. C. Such a process cannot be controlled when carried out on a commercial scale and is uneconomical since it requires up to a ten-fold excess of ethanolamine. Further, polymers having relatively high molecular weights cannot be prepared by this process.
The cotelomers obtained by this process were unsuitable for use as high quality binders in lacquers, adhesives, coatings, sealants or printing inks because of their fixed chemical composition and their very low molecular weight. In addition, their complete solubility is only achieved in unconventional solvents such as chloroform, methylene chloride or methanol.
It is an object of the present invention to provide new binders or binder components for lacquers, coatings, sealants, adhesives or printing inks which combine the advantages of the copolymers and polyimides known in the art, but which also overcome their disadvantages. This object may be achieved with the hydroxy functional copolymers having imide structural units described below and the process for their preparation.
The copolymers according to the invention described below were easily prepared, soluble in the commonly used lacquer solvents and suitable for many fields of application due to their molecular weight and their composition which may be varied within wide limits. The ease with which they can be prepared is due to the surprising finding that the special anhydride functional copolymers used as intermediate products can be modified by a reaction with suitable amino alcohols to yield uncrosslinked, readily soluble hydroxy functional copolymers containing imide structural units. This is surprising in view of the fact that it is known that organic polyanhydrides react with amino alcohols to form cross-linked, insoluble products (EP-A-48 128 (U.S. Pat. No. 4 452 948), DE-OS 1 769 466 (U.S. Pat. No. 3 878 151), DE-OS 2 214 450, DE-OS 2 405 981, U.S. Pat. No. 4 155 957 and DE-OS 2 214 451.